Many pressure sensors or transducers, such as micro-machined silicon pressure sensors, that require high reliability and stable performance for long periods of time are “absolute” pressure sensors that measure applied pressure relative to an integral reference vacuum reference chamber. Many are typically sealed within a hermetic hollow, outer body within which the reference vacuum resides. For example, a conventional high pressure silicon pressure sensor requires a hermetic attachment interface that connects an outer packaging element with an inner tube that supports the pressure sensing integrated circuit (IC) (i.e., pressure sensor “die”). Traditionally, the hermetic attachment interface includes solder or other adhesive material and takes the form of either a “butt” or a “sleeve” joint, as explained in greater detail below.
FIG. 1 shows a conventional absolute pressure sensor 100 having a sensor die 102 coupled to a glass support member 104. As illustrated, the glass support member 104 is adhered with an adhesive 106 to the package header 124. A package cover 108 includes a pressure inlet port 110. The sensor die 102 includes a sensor diaphragm 112 located between the pressure inlet port 110 and a vacuum reference chamber 114. In addition, the sensor die in this typical embodiment includes an active side 116 that faces away from the vacuum reference chamber 114. Electrical connections to and from the sensor die 102 are typically provided via bond wires 118 or an equivalent “lead frame” coupled to electrical interconnect pins 120. The pins 120 are positioned within insulated seals 122 that extend through a header portion 124 of the package cover 108. In this embodiment, the packaging material may be either metal or plastic since the only requirement for near-perfect hermeticity is for the seal between the sensor die 112 and the glass support member 104. That is required to maintain the integrity and hence stability of the reference vacuum. This configuration, however suffers mainly from the exposure of the active side (side with circuitry and interconnects on it) of the sensor IC die to atmospheric contaminants such as moisture, dust, and other fluidic and particulate contaminants, many of which are conductive to some extent. Such conductive contamination bridging between the electrical interconnects on the surface of the die and the pins and the package induces stray conduction paths that cause errors in the measurement. In an attempt to counter that, many sensors use a flexible silicone gel or parylene overcoat 125 as a barrier to contamination.
The silicone gel approach has several disadvantages, for example: 1) It mass-loads the top of the diaphragm causing increased sensitivity to G forces; 2) the gel is hygroscopic and slowly absorbs water over time thereby negating its potential benefit; 3) the gel deteriorates with time and environmental exposure, thereby changing its physical characteristics; and 4) the gel adds extreme thermal and pressure hysteresis to the measurement, thereby limiting its use for precision applications.
Parylene, being a gaseous-deposition ultra-thin coating performs orders of magnitude better in regard to most of the errors it induces relative to the silicone gel, however, it still suffers the following drawbacks: 1) It has limited environmental compatibility vs. time, temperature, and compatibility with the pressure media, especially oxygen and ozone; and 2) the parylene film deposited over the sensor diaphragm creates both thermal and pressure hysteresis due to the mismatch of the thermal coefficient of expansion between materials.
FIG. 2 shows a conventional differential pressure sensor 200 having a sensor die 202 coupled to a perforated glass support member 204. The perforated glass support member 204 is adhered with an adhesive 206 to a package 208. The package 208 includes a first pressure port 210 and a second pressure port 212 configured to permit a sensor diaphragm 214 to be displaced by the differential pressure acting on the diaphragm 214. Electrical connections to and from the sensor die 102 are provided via bond wires 218 coupled to electrical interconnect pins 220. The pins 220 are positioned within insulated seals 222 that extend through the header portion 224 of the package 208. In this configuration, the higher pressure must always be applied to pressure port 210 because the adhesive 206 used to attach the glass support member to the package is often insufficiently robust in tension to maintain a seal at the interface between package 224 and the glass support member 204 if the higher pressure were to be applied to port 212. In addition, a flexible silicone gel or parylene overcoat 225 operates as a barrier to contamination.
The main drawbacks for the conventional absolute and differential pressure sensors described above are the previously discussed performance, reliability and environmental robustness of the sensors due to their packaging approach. FIG. 3 schematically shows a portion of a pressure sensor 300 that alleviates at least some of the previously discussed problems since the pressure sensing die 302 is not coated with any material that would affect its performance and it also resides within a pristine vacuum environment 303.
In this embodiment, the sensor die 302 is coupled to an inner glass support tube 304. An adhesive or soldered “butt” joint 306 hermetically attaches the inner glass support tube 304 to a metallic package 308. The inner glass support tube 304 includes a passageway 310 where a pressurized media “P” loads the sensor die 302 when the sensor 300 is in operation. Loading the sensor die 302 with a positive pressure places the butt joint 306 in tension. In addition, torsional or bending loads applied to the sensor die 302 may tend to induce shear loads across the interface between the sensor die 302 and the inner glass support tube 304, for example the shear loads would be along lateral or radial axes as defined by the inner glass support tube 304. An overload of any one of these load conditions may result in a failure of the pressure sensor 300. Further, repeated loading of the sensor die 302 and resultant stressing of the solder or adhesive material may eventually degrade the structural integrity of the butt joint 306, causing a non-instantaneous degradation in sensor performance, and in some instances may lead to an instantaneous failure of the pressure sensor 300. This sensor configuration also has limited life in high vibration environments. This is due to the limited cross-sectional area of the attachment combined with the cantilevered configuration of the sensor and glass support tube assembly.
FIG. 4 schematically shows a portion of an improved pressure sensor configuration 400 having a sensor die 402 coupled to an inner glass support tube 404. A soldered “sleeve” joint 406 attaches the inner glass support tube 404 to an intermediate metallic sleeve 408 as described in U.S. Pat. No. 4,509,880 and is further located within a pristine vacuum environment 403. The inner glass support tube 404 includes a passageway 410 where a pressurized media “P” loads the sensor die 402 when the sensor 400 is in operation. Loading the sensor die 402 with a positive pressure (up the tube) places the sleeve joint 406 in shear and tension. Again and similar to the pressure sensor of FIG. 3, repeated loading of the sensor die 402 and stressing of the solder material in shear or tension may eventually degrade the structural integrity of the sleeve-type hermetic seal 406, causing a non-instantaneous degradation in sensor performance, or, in some instances, may lead to an instantaneous failure of the pressure sensor 400. This sleeve joint is stronger than the above-described butt joint in both vibration and pressure environments due to the increased support area of the joint, with resulting reliability improvements. In addition, the joint may include a metallic layer 412 to help the inner glass support tube 404 bond to the metallic sleeve 408. However, one drawback of the sleeve joint 406 is that it is still loaded in shear, which limits the amount of pressure up-the-tube to applications of 1000 psi or less.